[SI-LIST] Re: High speed signal-return path
- From: Ralph A Wilson III <wilson@xxxxxxxxxxxxxxxxxx>
- To: olaney@xxxxxxxx
- Date: Mon, 24 Sep 2007 06:38:51 -0500
Assuming
a) this is "Plane-Signal-Plane", and
b) both planes are contiguous over the course of the signal, and
c) the entry into and exit out of the signal layer is "managed"
with respect to Z0 and/or "short" lengths.
then these are equivalent - it matters not what the plane is (within
reason). I, however, don't trust assumptions "b" and "c". Split
power planes are too common these days, and "c" can be non-trivial.
I tend to be a fan of ground plane references - it makes the job
easier for my simple mind.
Ralph Wilson
Alcatel-Lucent
On 09/23/07 19:40, olaney@xxxxxxxx wrote:
> Keeping an eye on the original question, which was the choice between:
>=20
>> Option1:
>>L4 - Power
>>L5 - Signal
>>L6 =96 GND
>>
>> Option2:
>>L4 - GND
>>L5 - Signal
>>L6 =96 GND,
>=20
>=20
> ...are we agreed that these options are equivalent from a high frequenc=
y
> standpoint? And that the potential problems you mention regarding layo=
ut
> apply equally to power and ground plane implementations, and therefore
> are a separable problem from the choices shown above?
>=20
> Current always needs a return path. For a high frequency signal
> sandwiched between the planes of either option, the important concept i=
s
> that the return current flows on both planes, in a ratio determined by
> the proximity to each. For a trace centered between them, the return
> current in each plane will be half that on the signal trace. Of course=
,
> any DC component is restricted to returning via ground, but that is not=
> the same as the AC case. There is a classic EMC demo that shows the
> transition region between the two regimes, but basically, once the
> frequency is high enough for the bypass caps to make a difference, then=
> both planes in the microstrip sandwich participate as return paths, and=
> it doesn't matter whether both are ground planes, or one is power and o=
ne
> is ground, or both are power, all else being equal. In other words, th=
e
> assignment of a plane to power or ground is not, in and of itself, an S=
I
> issue.
>=20
> BTW, it is not entirely true that differential pairs have no ground
> return current. If a plane is close enough to affect the impedance of
> the pair, it will carry ground currents in both directions at opposite
> polarities, one for each trace. There will be a current null at the
> point midway between the traces and a peak underneath each trace. The
> sum will be zero, but the actual currents are not each zero because the=
> simple fact that the signals are balanced does not mean that the distan=
ce
> between the traces has no effect. For sufficiently distant planes or
> small enough separations (or both), the paired return currents might be=
> trivially small. For typical board stacks and trace pitches, Murphy sa=
ys
> they're not.
>=20
> Orin
>=20
> On Sun, 23 Sep 2007 10:11:03 -0700 (PDT) Jory McKinley
> <jory_mckinley@xxxxxxxxx> writes:
> This is interesting and hopefully not too long winded and off the
> track...........
>=20
> Correct me if I have this wrong but current always attempts to return t=
o
> its source no matter what frequency we are talking about, if it does no=
t
> then we have a problem such as radiation in the path. For AC signals t=
he
> return or image current is instantaneous to the traveling signal
> attempting to get back to the source through any low inductance path.=20
> This path may be the opposite line for differential it may be as you
> pointed out power or ground or both. However, there could be many
> hurdles for this return current, signal via's, reference plane splits,
> and as you mentioned improper decoupling to name a few. =20
> One of the issues on a board with a gnd/sig/gnd configuration is the
> potential long return loop for single ended signals routed on this laye=
r
> and potential issues with any unbalanced common mode differential noise=
=2E=20
> I will say that for "ideal" differential with well balanced decoupling,=
> and return vias the gnd/sig/gnd works but ideal is not that easy.
> -Jory
>=20
>=20
>=20
>=20
> ----- Original Message ----
> From: "olaney@xxxxxxxx" <olaney@xxxxxxxx>
> To: jory_mckinley@xxxxxxxxx
> Cc: raja.anand@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> Sent: Sunday, September 23, 2007 11:17:49 AM
> Subject: Re: [SI-LIST] Re: High speed signal-return path
>=20
>=20
> That makes sense for DC current, but the point is that the power and
> ground planes are shorted together in the RF sense by a properly
> implemented bypass network. Therefore, one is as good as the other for=
> RF. RF energy never reaches the power supply -- if it does, something =
is
> very wrong.
>=20
> Orin
>=20
> On Sat, 22 Sep 2007 22:11:06 -0700 (PDT) Jory McKinley
> <jory_mckinley@xxxxxxxxx> writes:
> Hello Orin,
> There is in that the return current has to do just that, return. So fo=
r
> single ended or common mode differential sourcing or sinking from power=
> or ground could make a significant difference, somewhat regardless of
> edge rate. All mute for ideal differential of coarse.
> -Jory
>=20
>=20
>=20
>=20
> ----- Original Message ----
> From: "olaney@xxxxxxxx" <olaney@xxxxxxxx>
> To: jory_mckinley@xxxxxxxxx
> Cc: raja.anand@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> Sent: Saturday, September 22, 2007 12:49:27 PM
> Subject: [SI-LIST] Re: High speed signal-return path
>=20
>=20
> We all know differential versus single ended, but "return current plane=
,
> not ground" needs explanation if you feel that there is some nontrivial=
> difference at high frequencies between power and ground planes.
> Orin
>=20
> On Sat, 22 Sep 2007 10:36:14 -0700 (PDT) Jory McKinley
> <jory_mckinley@xxxxxxxxx> writes:
> Yes, however every plane is a potential return current plane, not groun=
d,
> less important for true differential more important for single ended.=20
> How about any common mode noise on the differential signals, I would
> think Option 1 would allow for better potential isolation.
> -Jory
>=20
>=20
>=20
>=20
> ----- Original Message ----
> From: "olaney@xxxxxxxx" <olaney@xxxxxxxx>
> To: raja.anand@xxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Sent: Saturday, September 22, 2007 6:51:30 AM
> Subject: [SI-LIST] Re: High speed signal-return path
>=20
>=20
> Every power plane is a ground plane for high frequency purposes. There=
> should be no material difference between the two implementations. If
> there is, something is really wrong with the power supply bypassing
> strategy. The choice between stripline and microstrip is generally
> driven by the particulars of the board layout. There is no definitive
> choice of one over the other based on general principles, only as
> circumstances dictate.
>=20
> Orin Laney
>=20
> On Sat, 22 Sep 2007 11:12:19 +0530 "Raja Anand" <raja.anand@xxxxxxxxx>
> writes:
>=20
>>Hi All,
>> For high speed differential signals (3.125Gbps ), is it really=20
>>required to
>>take these signals between Gnd Planes. Which option should I go for=20
>>from
>>below if my stack up is symmetrical?
>>
>> Option1:
>>L4 - Power
>>L5 - Signal
>>L6 =96 GND
>>
>> Option2:
>>L4 - GND
>>L5 - Signal
>>L6 =96 GND
>>
>>Regards,
>>Raja
>>
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>>
>=20
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